The present invention relates to a method of smoothing the outer outline of a useful layer of a semiconductor material transferred onto a support during the fabrication of substrates for electronics, optics, or optoelectronics.
Currently, all substrates fabricated using techniques combining bonding by molecular bonding (known as “wafer bonding”) and transferring a useful layer onto a support have a zone known as a “peripheral ring”.
The ring is a zone located at the periphery of the support and for which transfer of the useful layer has not occurred, or when transfer has occurred, the ring is a zone in which the useful layer has been transferred partially, or has disappeared during subsequent treatment, due to its poor bonding to the support.
Accompanying FIGS. 1 and 2 are respective cross sectional and plan views of a substrate that is known to the skilled person under the acronym “SOI” meaning “silicon on insulator”.
FIG. 1 shows a support 1 of silicon onto which a composite layer 2 comprising a layer of silicon oxide 21 surmounted by a layer of silicon 22 has been transferred by molecular bonding.
The term “ring” 3 defines a substantially annular zone of support 1 onto which the composite layer 2 has either not been transferred or has been transferred poorly during transfer of the layer. The plan view (see FIG. 2) shows that the ring 3 varies in width, i.e., the lateral vertical side 200 of the composite layer 2 is irregular or jagged.
It should be noted that FIGS. 1 and 2 and the remaining figures are not drawn to scale both as regards to the thickness of the different layers and of the support and as regards the width of the ring.
The ring phenomenon occurs with other substrates, for example those known under the acronym “SICOI” meaning “silicon carbide on insulator” or under the acronym “SOQ” meaning “silicon on quartz”. Other multi-layer substrates such as those comprising gallium arsenide on silicon (AsGa/Si) also exhibit that ring.
Independently of the diameter of support 1, which can vary, for example, from 2 inches (50 millimeters (mm)) for silicon carbide to 12 inches (300 mm) for certain silicon substrates, the ring 3 is regularly a few millimeters in width. Further, the width can vary, i.e., it can be 1 mm on one side of the substrate and can reach 4 mm on the other side, for example.
The appearance of the ring during layer transfer has a variety of origins, as discussed below. Particular mention can be made of the existence of chamfers on the substrates used, variations in bonding energy, the bonding techniques employed, and finally certain aggressive steps in substrate fabrication methods.
In order to explain the appearance of the ring, reference is made to the accompanying FIG. 3 which is a diagrammatic cross section of a portion of the sides of two substrates bonded together by molecular bonding. A source substrate 4 from which the future useful layer is to be cut, and a support substrate 5 intended to receive the useful layer are shown. This figure illustrates the prior art.
In the remainder of the description and drawings, the substrates are assumed to be circular in shape as this is the shape encountered most frequently. However, they can have other shapes.
The source substrate 4 has two opposite faces that are parallel; in FIG. 3 reference numeral 400 designates a “front” one of the faces. The front face 400 is intended for bonding onto the support 5. The source substrate 4 has a side 41 which is perpendicular to the plane of the front face 400.
Further, the substrate 4 has undergone treatment that forms a zone of weakness 42 that defines two portions, a rear portion and a useful layer 43 intended for subsequent transfer to the support 5.
Throughout the remainder of the description and claims, the expression “useful layer” designates a transferred layer of thickness that depends on whether it is obtained, for example, by a method of implanting atomic species or by abrasive polishing and/or chemical etching as is described below.
Currently, the substrates used both as the source substrate and as the support substrate are commercially available substrates satisfying standardized requirements (for example SEMI standards for a silicon substrate). These requirements are mainly concerned with ensuring that the substrates can be accepted by the equipment of as wide a range of users as possible.
According to those standards, at the intersection between the side 41 and the front face 400, the substrate 4 has an annular primary chamfer 44 or primary drop making a large angle α (close to 45°) with the plane of the front face 400 and more precisely with the rigorously flat central zone, as will be explained below. The primary chamfer 44 extends over a width L in the radial direction. The width L varies from 100 micrometers (μm) to 500 μm depending on the different substrates. The primary chamfer 44 is intended to limit the risk of mechanical rupture and notching of the source substrate 4.
In a similar manner to that just described for the source substrate 4, the support substrate 5 has a front face 500, a side 51 and a primary chamfer 54.
When substrates 4 and 5 are bonded to each other, bonding does not occur at chamfers 44 and 54 because of the magnitude of angle α. The width of the ring can thus be expected to correspond to the width L of the primary chamfers 44 and 54. It has been shown, however, that it is even wider in practice.
It has been observed that the front face 400 of substrate 4 actually has two zones, namely a first rigorously flat zone 40 located substantially at the center of the substrate 4 and hereinafter termed the “flat central zone” and a second zone 45 surrounding the first zone.
The second zone 45 is a secondary annular chamfer or secondary drop forming an angle β with the plane of the central zone 40. This angle β is very small, generally less than 1°, so the secondary chamfer 45 actually constitutes a slight deviation from the plane of the central zone 40. The secondary chamfer 45 extends between the flat zone 40 and the primary chamfer 44.
Throughout the remainder of the description and claims, the expression “flat” means a flatness that is suitable for bonding, and the expression “central zone” designates a zone located substantially at the center of the front face of the substrate but which can, however, be located slightly excentrically on the face.
It should be noted that FIG. 3 and the following figures are only diagrammatic in nature and the magnitude of angle β has been considerably exaggerated in the figures for clarification purposes.
More precisely, the secondary chamfer 45 constitutes a drop that is less sharp than the primary chamfer 44 and which appears during the various substrate shaping steps (lapping, polishing, chemical etching). These steps producing an etching and material-removal effect that is greater on the substrate side. The secondary chamfer 45 is not subject to standards. Its width L′ taken in a radial direction varies from about 500 μm to 3000 μm. The secondary chamfer 45 is a poorly defined zone of dimensions that are neither controlled or completely reproducible from one substrate to another. Further, the value of angle β also fluctuates, and so secondary chamfer 45 is not flat as shown diagrammatically in FIG. 3 but can be domed or irregular in places.
As a result, in reality (and in contrast to the diagrammatic representation of the figures), the side of the source substrate 4 is not formed by a plurality of beveled slopes but has a somewhat rounded shape, i.e., without edges between the secondary chamfer 45 and the primary chamfer 44 or between the primary chamfer and the side 41.
In a manner similar to that just described for the source substrate 4, the support substrate 5 has a flat central zone 50 and a substantially annular secondary chamfer 55. The annular secondary chamfer 55 has the same irregularities as the secondary chamfer 45.
Molecular bonding is a technique that cannot tolerate non-planar surfaces. Thus, the existence of the secondary chamfers 45, 55 results in poor bonding and layer transfer in that zone, resulting in the appearance of a peripheral ring.
A second reason for the appearance of the ring is that in general, the bonding energy between two facing faces reduces on moving from the center to the side of a substrate with the degree of bonding varying. In other words, the bonding energy is always lower at the periphery of the substrates.
The bonding energy also fluctuates as a function of parameters such as roughness, flatness and the chemical nature of the surfaces in contact, the presence of particles, etc. The parameters can also vary in a less controlled manner at the sides of the substrates.
Further, the bonding energy also depends on the deformation force on the source substrate and on the support substrate when they are pressed against each other. When bonding with bond initiation on one side, it is observed that the bonding energy is lower over the final portion of the bonding wave than over the initial portion or the central portion. As a result, in the final bonding zone, the ring is often more irregular and/or wider and/or more fragile.
Finally, a third possible cause for formation of the ring is the use of certain aggressive steps taken during the substrate fabrication methods.
Methods of fabricating substrates known under the acronym BESOI (bond and etchback silicon on insulator) bond a source substrate onto a support substrate, at least one of the faces of the source substrate being coated with a layer of oxide. The exposed surface of the source substrate then undergoes an abrasive polishing and/or chemical attack etching treatment followed by polishing until the source substrate becomes a useful layer.
In that type of method involving chemical attack (with the risk of partial delamination of the bonding interface), oxidation affecting the lateral and frontal portions of the source substrate, and mechanical abrasive polishing forces, both tend to enlarge the ring.
Similarly, in methods involving detachment of the layer by fracture along a zone of weakness, it has been observed that around the sides, detachment tends to occur at the bonding interface and not at the zone of weakness, resulting in the formation of an annular ring with a large surface area.
Referring again to FIG. 3, in the particular case of the zone of weakness 42 being formed by hydrogen implantation, it has been shown that, during subsequent treatment to detach the useful layer 43 from the remainder of the source substrate 4, expansion of hydrogen bubbles exerts a substantially perpendicular force on the plane of the secondary chamfer 45. In that zone, the force is not compensated by the existence of a surface that is in direct contact with and against which the secondary chamfer can press, since the secondary chamfer 55 is spaced from the secondary chamfer 45 by an angle 2β. Thus, bubbles are formed at the surface of the secondary chamfer 45 and those bubbles further reduce the bonding force between the secondary chamfers 45 and 55.
A number of disadvantages are associated with the existence of the peripheral ring.
First, the side of the transferred useful layer defining the ring is fragile and can crumble away during the treatments to which the final substrate is subjected. In addition to the disappearance of precious millimeters of useful layer during the component fabrication, crumbling of the layer produces particles that are a source of impurities that severely affects the fabrication yields of circuits formed from the components. By way of example, a particle with a diameter on the order of 0.1 μm is sufficient to destroy a 0.25 μm circuit.
Finally, the ring is irregular, as explained above, and its width can vary from about 1 mm to 4 mm from one side of the substrate to the other, which causes problems concerning reproducing the different steps in an industrial process when such a substrate is used in a production facility.
A goal of the invention is to overcome the disadvantages described above and in particular to render smooth and regular the side face or rim of the useful layer transferred onto a support during a layer transfer method.